Covariant evolution of perturbations during reheating in two-field inflation

Understand

We develop a covariant method for studying the effects of a reheating phase on the primordial adiabatic and isocurvature perturbations in two-field models of inflation.

• To model the decay of the scalar fields into radiation at the end of inflation, we introduce a prescription in which radiation is treated as an additional effective scalar field, requiring us to extend the two-field setup into a three-field system.
• In this prescription, the coupling between radiation and the scalars can be interpreted covariantly in terms geometrical quantities that parametrize the evolution of a background trajectory in a three-field space.
• In order to obtain concrete results, we consider two scenarios characterized for having unsuppressed isocurvature fluctuations at the end of inflation: (1) canonical two-field inflation with the product exponential potential, which sources a large negative amount of non-gaussianity and, (2) two-field inflation with an ultra-light field, a model in which the isocurvature mode becomes approximately massless, and its interaction with the curvature perturbation persists during the entire period of inflation.